1. Field of the Invention
The present invention relates to a substrate for an organic light-emitting device, a method of fabricating the same, and an organic light-emitting device having the same, and more particularly, to a substrate which can improve the light extraction efficiency of an organic light-emitting device while realizing an intended level of transmittance, a method of fabricating the same, and an organic light-emitting device having the same.
2. Description of Related Art
In general, an organic light-emitting diode (OLED) includes an anode, a light-emitting layer and a cathode. When a voltage is applied between the anode and the cathode, holes are injected from the anode into a hole injection layer and then migrate from the hole injection layer through a hole transport layer to the organic light-emitting layer, and electrons are injected from the cathode into an electron injection layer and then migrate from the electron injection layer through an electron transport layer to the light-emitting layer. Holes and electrons injected into the light-emitting layer recombine with each other in the light-emitting layer, thereby generating excitons. When the excitons transit from an excited state to a ground state, light is emitted.
Organic light-emitting displays including an OLED are divided into a passive matrix type and an active matrix type depending on the mechanism that drives the N*M number of pixels which are arranged in the shape of a matrix.
In an active matrix type, a pixel electrode which defines a light-emitting area and a unit pixel driving circuit which applies a current or voltage to the pixel electrode are positioned in a unit pixel area. The unit pixel driving circuit has at least two thin-film transistors (TFTs) and one capacitor. Due to this configuration, the unit pixel driving circuit can supply a constant current irrespective of the number of pixels, thereby realizing uniform luminance. The active matrix type organic light-emitting display consumes little power, and thus can be advantageously applied to high definition displays and large displays.
When light generated by an OLED having an internal emission efficiency of 100% exits through, for example, a transparent conductive film made of indium tin oxide (ITO) and a glass substrate, its efficiency is about 17.5% according to Snell's Law. This decreased efficiency has a significant effect on the reduction in the internal and external luminous efficiencies in the organic light-emitting device using the glass substrate. In order to overcome this, the #transmittance efficiency is increased by increasing optical light extraction efficiency. Accordingly, a number of methods for increasing the optical light extraction efficiency are underway.
Light extraction techniques of the related art include a technique of treating a surface having a texture structure on a glass plate, the technique of applying microspheres to a glass surface on which ITO is deposited, the technique of applying micro-lenses on the glass surface on which ITO is deposited, the technique of using a mesa structure, the technique of using silica aerogel on ITO and the glass surface, and the like. Among these techniques, the technique of using silica aerogel had the effect of increasing the quantity of light by 100%. However, silica aerogel is very sensitive to moisture and is unstable, thereby resulting in the reduced longevity of the device. Accordingly, it was impossible to commercially use this technique.
In addition, although the technique of using the micro-lenses or mesa structure increased the external light efficiency, the fabricating cost was greatly increased. This consequently causes the problem of low practicability. In addition, in the technique of using microspheres, no increase in the external luminous efficiency appeared but only the wavelength was changed due to the dispersion of light. Therefore, the method of using the texture structure that has brought the efficiency increase of 30% to the organic light-emitting device is most advantageous in terms of the longevity and cost of the device. However, since glass is amorphous, it is very difficult to form the texture structure having a certain shape on the glass plate. In addition, even if the texture is formed on the glass plate, the flatness is lowered by the texture. Consequently, the texture structure is also formed on the surface of the anode that adjoins to the glass plate, whereby leakage current occurs. This consequently creates many problems in the structure or process. For example, when the texture structure is applied for internal light extraction, an additional planarization film is required.
The information disclosed in the Background of the Invention section is provided only for better understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.